As disclosed in JP-A-2005-127050, a vehicle may have a welcome function in which it is detected that a user carrying a portable device has approached the vehicle, and a vehicle-mounted apparatus is operated to welcome the user to the vehicle. For example, the welcome function is realized by use of a keyless entry system or a passive entry system.
Specifically, a vehicle-mounted transmitter mounted in the vehicle intermittently transmits a response request signal at predetermined intervals. When a user carrying a portable device approaches the vehicle, the portable device receives the response request signal, and responds with a response signal. When the vehicle-mounted transmitter mounted in the vehicle receives the response signal, for example, a welcome light provided in a side view mirror is turned on to illuminate the ground in the vicinity of a door, and welcomes the user to the vehicle.
The transmission of a response request signal from the vehicle-mounted transmitter is performed by a polling method. For this reason, when the amount of stop time of an engine of the vehicle is increased, and the vehicle-mounted transmitter intermittently transmits a response request signal, electrical power is continuously consumed, and a battery of the vehicle, which is a power supply, is discharged, which is a problem. In the system disclosed in JP-A-2001-098810, when the voltage of a battery is decreased to be lower than the voltage required to guarantee the driving of a door lock mechanism, or when a function stop switch provided in a portable device or a vehicle is operated, the transmission of a response request signal from a vehicle-mounted transmitter is stopped.
As described in JP-A-2008-038514, multiple vehicle-mounted transmitters may be respectively provided for seats of a vehicle. In the system disclosed in JP-A-2008-038514, in order to efficiently search for a portable device, response request signals are efficiently transmitted according to an intermittent output control pattern in which the amount of time, for which two or more (but less than the total number of vehicle-mounted transmitters) of the vehicle-mounted transmitters transmit response request signals at the same time, is determined.
A transmission output level is required to be adjusted such that the field strength of a response request signal transmitted from a vehicle-mounted transmitter does not exceed a specified value specified in the Radio Law at any position in the vicinity of a vehicle. In the system disclosed in JP-A-H11-074802, a receiver measures the received field strength of radio waves transmitted from a transmitter, and a correction value for a transmission output level is calculated by comparing the measured value with a reference value stored in advance. The transmission output level is changed based on the correction value, and a transmitter transmits radio waves based on the changed transmission output level.
In contrast, JP-A-2015-021249 discloses a remote operation system which detects a specified operation performed by a user via a portable device (electronic key), and allows or executes the remote operation of a vehicle-mounted apparatus when the operation is determined to be valid. In the system disclosed in JP-A-2015-021249, the portable device determines the validity of the specified operation performed by the user, based on a detection value from an acceleration sensor. In a case where the amount of change between received signal strength when a remote operation is started and received signal strength when the remote operation is ended is calculated, and in a case where the amount of change is a threshold value or less, it is determined that a user performs the remote operation, and thus, the portable device allows the remote operation. When a vehicle-side apparatus receives a notification for the start of an operation from the portable device, the vehicle-side apparatus further shortens the poling period of radio waves transmitted from a vehicle-mounted transmitter compared to that of a normal period. When a predetermined amount of time has elapsed, or when the vehicle-mounted apparatus receives a notification for the end of the operation or the stop of the operation from the portable device, the vehicle-mounted apparatus returns the poling period to the original value.